1. Field of the Invention
The invention relates to a catalyst composition for polymerizing olefins, particularly alpha-olefins, a method for producing such a catalyst and to a method of polymerizing olefins with such a catalyst. More particularly, the invention relates to a high activity catalyst composition which can be used to produce either high density, high molecular weight olefin polymers, or linear, low density polyolefins, both having molecular weight distribution which can be varied by the polymerization process conditions. The invention is also directed to the polymerization process utilizing such a catalyst composition.
2. Description of the Prior Art
Linear low density polyethylene (LLDPE) polymers possess properties which distinguish them from other polyethylene polymers, such as ethylene homopolymers. Certain of these properties are described by Anderson et al, U.S. Pat. No. 4,076,698. The LLDPE polymers are usually prepared from alpha-olefins in the presence of catalysts commonly referred to as Ziegler or Ziegler-Natta catalysts. Such catalysts usually contain various combinations of a magnesium compound, a compound of a transition metal, e.g., titanium or vanadium, and a co-catalyst, e.g., an aluminum alkyl.
Graff, U.S. Pat. No. 4,173,547, Stevens et al, U.S. Pat. No. 3,787,384, Strobel et al, U.S. Pat. No. 4,148,754, and Ziegler, deceased, et al, U.S. Pat. No. 4,063,009, each describe various polymerization processes suitable for producing forms of polyethylene other than linear low density polyethylene.
Stevens et al, U.S. Pat. No. 3,787,384, and Strobel et al, U.S. Pat. No. 4,148,754, describe catalysts prepared by first reacting a support (e.g., silica containing reactive hydroxyl groups) with an organomagnesium compound (e.g., Grignard reagent) and then combining this reacted support with a tetravalent titanium compound.
Ziegler, deceased, et al, U.S. Pat. No. 4,063,009, describe a catalyst which is the reaction product of an organomagnesium compound (e.g., an alkylmagnesium halide) with a tetravalent titanium compound. The reaction of the organomagnesium compound with the tetravalent titanium compound takes place in the absence of a support material.
Thus, catalysts used for producing LLDPE resins usually contain a transition metal or a compound thereof and magnesium or a compound thereof. Such catalysts are intended for the production of LLDPE polymers having relatively low density (0.930 g/cc or less) and narrow molecular weight distribution (low MFR), since low MFR values are thought to be responsible for good strength properties of the films made from such polymers.
With some catalysts disclosed in the patents and publications discussed below, high yields of good quality, high molecular weight, polymers of ethylene and other olefins may be produced. Generally, these are linear polymers of high density, i.e., 0.930 g/cc and above, referred to in the art as high density polyethylene (HDPE), with the molecular weight of the polymers falling within a wide range of 2,000 to 300,000 or more. The density and molecular weight characteristics of these polymers render them satisfactory for many uses and they have, in fact, in recent years found extensive commercial use in a variety of applications. However, the polymer products obtained are not always suitable for specialty uses because they do not have the desired molecular weight distribution. Generally, high density and high molecular weight polymers are preferred for their superior strength characteristics. However, such polymers tend to have narrow molecular weight distribution (as evidenced by low values of melt flow ratio, MFR,) and relatively low values of high load melt index (I.sub.21) which renders them difficult and expensive to process, i.e., they require a high extrusion pressure for processing, thereby resulting in low productivity and high cost per unit of product. Therefore, it is desirable to provide a catalyst producing high density, low I.sub.21 polymers having comparatively broad molecular weight distribution (high MFR) since such polymers can be easily processed and they have satisfactory strength properties.
A vanadium-containing catalyst, used in conjunction with triisobutylaluminum as a co-catalyst, is disclosed by W. L. Carrick et al, in Journal of American Chemical Society, Volume 82, page 1502 (1960) and Volume 83, page 2654 (1961).
Altemore et al, U.S. Pat. No. 3,899,477, disclose a catalyst comprising a titanium halide, a vanadium halide and an organoaluminum compound. The catalyst is admixed with a mixture of an alkylaluminum sesquiethoxide and a trialkylaluminum prior to the use thereof in the polymerization of ethylene to produce high molecular weight and broad molecular weight distribution polymers. The catalyst may be supported on a support by preparing a solution or a slurry thereof and admixing it thoroughly with the support.
Ort, U.S. Pat. No. 3,956,255, discloses a supported catalyst composition made by depositing on a previously-treated silica gel a compound of vanadium, a trialkyl aluminum, and an alkylaluminum alkoxide. The silica is treated with an alkylaluminum, and alkylaluminum alkoxide or mixtures thereof to provide a catalyst of high activity.
Best, U.S. Pat. No. 4,607,019, discloses an olefin polymerization catalyst composition producing high molecular weight and broad molecular weight distribution polyolefins comprising a vanadium-containing catalyst component and an aluminum alkyl co-catalyst. The vanadium-containing catalyst component is prepared by contacting an inert support with an organoaluminum compound a halogenating agent and a vanadium compound.
Best, U.S. Pat. Nos. 4,579,835 and 4,634,751, disclose vanadium-based olefin polymerization catalyst compositions comprising a support material, treated with an organoaluminum compound, an alkyl halide and a vanadium compound, activated with an alkylaluminum cocatalyst.
Best, U.S. Pat. Nos. 4,578,374 and 4,579,834, disclose vanadium- and magnesium-containing supported olefin polymerization catalyst compositions.
Thus, heretofore two separate and distinct catalytic systems were necessary for the production of either the HDPE or the LLDPE products. It would be desirable, however, to provide a single catalyst composition capable of producing either the HDPE or the LLDPE products.